Since it's cold and snowy, I am spending my idle time wondering about maintenance once I dig my XJ8 out when warmer weather returns....

The shortcut secondary tensioner replacement procedure looks pretty straightforward and a lot less work than removing the entire front end of the engine. What I was wondering was, has anyone changed out the secondary chain at the same time? Once the flats on the camshafts are aligned, and then immobilized, it looks like it should be possible to grind off one or more pins of secondary chain link(s) (with say a Dremel tool) , and fish the chain out from behind the primary. Then fit a new chain, with master link, in its place.

Is this feasible?

 

Chains with repair links have been used over the years but most professionals frown upon them. The issue is that the secondary chain is between the primary chain and the intake cam. Since everyone is removing the exhaust cam these days to replace the tensioner why not remove the intake cam and remove and install the new chain from the back?

Sorta get the cam flats aligned.

Use a zip tie to keep the primary chain and the intake sprocket aligned.

Remove the exhaust sprocket and separate the sprocket and chain

Carefully remove the intake cam based on numerous cautionary threads on this forum.

Slip the new chain around the intake cam from the back and the exhaust from the front

Install the new tensioner

Carefully bolt the cam back down

Install the cam alignment tool

Bolt up the exhaust sprocket nut to torque specs

Did all that in my head. Have no idea if it will work. You will have to handle the details.

 

Let me be sure I understand this....

Only the intake cam sprocket stays fastened to its cam; the exhaust is removed? After the secondary tensioner is removed, looks like the old chain gets pulled off from the exhaust cam sprocket, and then removed by working it down the length of the (now disconnected) intake cam to the rear. Yes? And the zip tie keeps the intake cam synchronized with the primary drive chain.....

With both cams unbolted from the cylinder head, is it possible to get the cams close enough together to remove the chain from the exhaust sprocket, without disconnecting the exhaust sprocket from the exhaust cam? That would greatly simplify the process, as it would eliminate the need for a cam alignment tool - one could make reference marks on the intake and exhaust sprockets, and then count links in between to establish cam alignment between the two.

 

I perceive that you grasp the concept. As I mentioned, I have never done this . . . at least not this way. Removing the intake cam will PROBABLY provide enough slack to remove the chain.

Making marks or counting links is just not precise enough for me. Not only does the cam alignment tool lock down the cam flats but properly installing the exhaust sprocket and torquing the bolt includes tensioning the chain.

An 1 - 2% error in intake/exhaust cam alignment would surly produce that much, or more, reduction in engine efficiency.

 

Thanks, test point.

I'll have to stew on this for awhile. It's going to be quite a few weeks up here before there will be any maintenance on the car.

One further question - in your original comment above you say
"remove the intake cam based on numerous cautionary threads on this forum"
Could you point me to one or more of these? Not sure what to be cautious about. Thx.

 

There is one problem with moving/disturbing the Intake cam that I can think of and that is if the primary tensioners have been replaced with the newer metal units.

The newer metal primaries are of the RATCHET TYPE and once extended, they WILL NOT retract until the release lever on the unit is moved manually. The way they are installed is to set the chains, guides and tensioners THEN remove the locking pin to release the ratchet.

If the intake sprocket is moved (tipped downward) and the chain gets slack in it, then the ratchet will extend to compensate. You will never be able to pull the chain back into position until the front cover is removed for access to the ratchet release.

The old plastic primaries are hydraulic ONLY and they should be OK as they are locked in place and that is probably what you have.

If the secondary chains are not damaged or stretched, then I would just replace the secondary tensioners.

Not trying to scare you, just be aware of what you have.

bob gauff

 

Bob-
Well, you scared me! Can one tell the type of primary tensioner with just the valve covers off?

And, of the metal type, where is the ratchet release? Any hope of using some kind of tool (cleverly bent coat hanger?) to get down there and release the ratchet?

 

And, for that matter, how can you tell whether or not the secondary chain is stretched?

 

Here is a link to the original thread on removing the exhaust cam to replace the tensioner: https://www.jaguarforums.com/forum/s...ad.php?t=37415

In it is a couple comments about care in removing and installing a cam.

I would be very surprised if anyone would go to the trouble of changing the primary tensioners without changing the secondary also.

 

My 2¢ worth would be to go with the advice of Motorcarman and simply replace the secondary tensioners. Then, If the chains are noisy and need replacing, you could reconsider your next move. I think you run a good chance of having the primary chain slip off the guide if you remove the intake cam. If that happened you could compound your problems significantly. Like Test Point, I am just throwing these thoughts together, but what would you do if the primary chain came off the guide? Turning the engine for valve timing purposes would be quite risky.

I have not looked, but are there any threads from anyone who tried this? I considered it and then chickened out and pulled the timing cover off.

 

Chain stretch would be your problem!
The only reason why the "zip-tie-method" works (and if done properly is not messing up the timing) is that you keep the original secondary chain and re-install everything in exact the same position.
Doing this the alignment between intake and exhaust cams will stay exactly the same even with new tensioners!

If you try to change the secondary chain the intake-exhaust cam alignment will change!
So even if you are gonna try this method you'll have to undo the sprocket drive from the cams and you'll need the alignment tool(s) to get the timing right.

David

 

I have been thinking about some of the comments here and on other threads, about camshaft misalignments resulting from skipping a tooth or from chain stretch.

I’m not sure exactly how many teeth the secondary sprocket has, or what the dimensions of the cam components are. But for the sake of doing some simple estimates I will assume:
Secondary chain sprockets have mean pitch diameter of D = 3”, have 20 teeth, and are Z = 7” apart. (just from looking at JTIS or thread photos)

So if the chain skips a tooth, the affected cam would be off by (360 deg)/20 = 18 degrees. That amount should be evident to the eye.

What about chain stretch (wear, really) ? We can estimate it based on how far the tensioner pushes up the slack side of the chain. For the sake of argument, suppose that a new chain has no slack, so that the tensioner cannot push the chain up at all. And also, for the sake of argument, suppose that for a worn chain the chain tensioner pushes the slack side up y = 0.5”. (A plausible number – hard to imagine it being much more than that) The extra chain (hypotenuses of the triangles vs straight across) compared to the straight-across length is approximately y^2/Z = 0.020”. But that slack has to be spread along the whole chain length, which for these presumed dimensions would be L = 2*7 + pi*3 = 23.42” So the fractional increase in the length of the chain is 0.020 / 23.42 = 0.00085” per inch of chain. Now 7” of the stretched chain is between the pulleys, representing a change in the spacing of the pulley cogs of 7*.00085 = 0.006”. That represents an angular error in cam position of 0.006/1.5 radians = 0.22 degrees. Which is probably insignificant. Or not??


I think the conclusion from this must be that unless the chain looks like it is about to break, or there is so much slack the tensioner is at its limits, there is no reason to swap the secondary chain out. (unless I made some math errors!)

 

I tried the exhaust cam only method of changing the secondary tensioners before I eventually went the whole nine yards. While it is possible, having three hands would be helpful. There is also not much "slack" when you come to the putting everything back in place.

As to the angles and such, the thing that happens in a proper job is that you align the cam flats and then apply tension to the drive sides of the primary and secondary so they are locked against the direction of pull. So it isn't just the alignment of the cams, but also the application of tension while tightening the sprockets that sets it all right. My flats moved very little after getting the chains tensioned properly and removing the locking tool.

 

Boomer, Where were you when I needed help for my math exam?

I would tend to agree with your conclusion. With a less scientific method, I ended up changing my chains through the process of elimination. In my first attempt at fixing my engine after tensioner failure and a skipped tooth, I changed only the secondary tensioners, which of course is considerably less work than the entire chain job. Oddly, for about 3 weeks it seemed to be just fine, then I got chain noise again in the left secondary chain. That prompted me to replace all the chains, guides and primary tensioners. I laid the old secondary next to the new one and had them as taught as I could get them, but could not see any noticeable difference. I had me a bit worried, but after replacing everything with new components, the noise was gone.

As a point of interest, the old plastic primary tensioners did not show any signs of abnormal wear or damage. The guide rails had some cracks in them however. This is apparently consistent with what others have found. The Jaguar dealer told me the new metal tensioners do not have quite as much stroke as the original plastic ones.

Changing all of them brings some peace of mind, but I probably would have stopped at the secondary tensioners if the chain noise did not recur.

I concur with Jaginblack, pre-tensioning the chains with the cam flats locked in place is the way to go.